How do I get started with multi-agent reinforcement learning?
After checking the Internet, you will probably find several resources such as

https://github.com/mohammadasghari/dqn-multi-agent-rl
https://rlss.inria.fr/files/2019/07/RLSS_Multiagent.pdf
https://arxiv.org/abs/2011.00583
and others.

Try to understand the principles first (see above). After some reasonable amount of coding you can adapt OpenAI gym.
Good luck!